Pin tailwindcss and check the committed stylesheet against it (closes #231)

make css ran whatever tailwindcss binary was on the host's PATH, so the committed stylesheet depended on the machine that built it, and nothing noticed when a template used a class the stylesheet lacked. make css now runs the standalone tailwindcss v4.2.1, pinned by sha256, in a Dockerfile stage, and a check stage, run by make check and required by the image build, fails when the committed static/css/tailwind.css differs from what the templates need, showing the differing rules. input.css names its sources. The unused .btn-text is removed and the stylesheet regenerated, dropping only unused rules. The README has a Stylesheet section.

Model: opus-5-5

script/check

@@ -1,8 +1,8 @@
 #!/bin/sh
-# script/check: run all checks (test, lint, fmt-check). Our own
+# script/check: run all checks (test, lint, fmt-check, css-check). Our own
 # extension to scripts-to-rule-them-all.
 # Writes only the ignored static/js/alpine.min.js, through script/test.
-# Generic: usually needs no adaptation.
+# Generic, apart from css-check.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -11,6 +11,7 @@ main() {
     "$SCRIPT_DIR/test"
     "$SCRIPT_DIR/lint"
     "$SCRIPT_DIR/fmt-check"
+    "$SCRIPT_DIR/css-check"
 }
 
 main "$@"